scholarly journals Derivation of bedrock topography measurement requirements for the reduction of uncertainty in ice sheet model projections of Thwaites Glacier

2021 ◽  
Author(s):  
Blake A. Castleman ◽  
Nicole-Jeanne Schlegel ◽  
Lambert Caron ◽  
Eric Larour ◽  
Ala Khazendar
Keyword(s):  
Sea Level Rise ◽  
Sea Level ◽  
Earth Science ◽  
Regional Scale ◽  
Ice Sheet ◽  
Model Space ◽  
Future Evolution ◽  
Continental Scale ◽  
Bedrock Topography ◽  
Grounding Line

Abstract. Determining the future evolution of the Antarctic Ice Sheet is critical for understanding and narrowing the large existing uncertainties in century-scale global mean sea level rise (SLR) projections. One of the most significant glaciers/ice streams in Antarctica, Thwaites Glacier, is at risk of destabilization and, if destabilized, has the potential to be the largest regional-scale contributor of SLR on Earth. This is because Thwaites Glacier is vulnerable to the marine ice sheet instability, as its grounding line is significantly influenced by ocean-driven basal melting rates, and its bedrock topography retrogrades into kilometer deep troughs. In this study, we investigate how bedrock topography features influence the grounding line migration beneath Thwaites Glacier when extreme ocean-driven basal melt rates are applied. Specifically, we design experiments using the Ice-Sheet and Sea-level System Model (ISSM) to quantify the SLR projection uncertainty due to reported errors in the current bedrock topography maps that are often used by ice sheet models. We find that spread in model estimates of sea level rise contribution from Thwaites glacier due to the reported bedrock topography error could be as large as 21.9 cm after 200 years of extreme ocean warming. Next, we perturb the bedrock topography beneath Thwaites Glacier using wavelet decomposition techniques to introduce realistic noise (within error). We explore the model space with multiple realizations of noise to quantify what spatial and vertical resolutions in bedrock topography are required to minimize the uncertainty in our 200-year experiment. We conclude that at least a 2 km spatial and 8 m vertical resolution would independently constrain possible SLR to ±2 cm over 200 years, fulfilling requirements outlined by the 2017 Decadal Survey for Earth Science. Lastly, we perform an ensemble of simulations to determine in which regions our model of Thwaites Glacier is most sensitive to perturbations in bedrock topography. Our results suggest that the retreat of the grounding line is most sensitive to bedrock topography in proximity to the grounding line's initial position. Additionally, we find that the location and amplitude of the bedrock perturbation is more significant than its sharpness and shape. Overall, these findings inform and benchmark observational requirements for future missions that will measure ice sheet bedrock topography, not only in the case of Thwaites Glacier but for Antarctica on the continental scale.

scholarly journals Exploration of Antarctic Ice Sheet 100-year contribution to sea level rise and associated model uncertainties using the ISSM framework

2018 ◽  
Author(s):  
Nicole-Jeanne Schlegel ◽  
Helene Seroussi ◽  
Michael P. Schodlok ◽  
Eric Y. Larour ◽  
Carmen Boening ◽  
...  
Keyword(s):  
Sea Level Rise ◽  
Sea Level ◽  
Ice Sheet ◽  
Snow Accumulation ◽  
Ice Shelf ◽  
Amundsen Sea ◽  
Antarctic Ice Sheet ◽  
Future Evolution ◽  
Continental Scale ◽  
Order Of Magnitude

Abstract. Estimating the future evolution of the Antarctic Ice Sheet (AIS) is critical for improving future sea level rise (SLR) projections. Numerical ice sheet models are invaluable tools for bounding Antarctic vulnerability; yet, few continental scale projections of century-scale AIS SLR contribution exist, and those that do vary by up to an order of magnitude. This is partly because model projections of future sea level are inherently uncertain and depend largely on the model's boundary conditions and climate forcing. Here, we aim to improve the understanding of how such uncertainties affect ice sheet model simulations. With use of Monte-Carlo style uncertainty quantification techniques embedded within the Ice Sheet System Model (ISSM) framework, we assess how uncertainties in snow accumulation, ocean induced melting, ice viscosity, basal friction, bedrock elevation, and the presence of ice shelves, impact continental scale 100-year projections of AIS sea level contribution. Overall, we find that AIS sea level contribution is strongly affected by grounding line retreat, which is driven by the magnitude of ice shelf basal melt rates and by errors in bedrock topography. In addition, we find that over 1.2 meters of AIS global mean sea level contribution over the next century is achievable, but not likely, as it is tenable only in response to unrealistically large melt rates and instantaneous continental ice shelf collapse. Regionally, we find that under an endmember 100-year warming scenario generalized for the entire ice sheet, the Amundsen Sea Sector is the most significant source of model uncertainty (1032 mm 6σ spread). This region also has the largest potential for future sea level contribution (297 mm). In contrast, under a more plausible scenario informed regionally by literature and model sensitivity studies, the Ronne basin has a greater potential for local increases in ice shelf basal melt rates. As a result, under this more likely scenario where warm waters reach the continental shelf under the Ronne ice shelf, it is the Ronne basin, particularly the Evans and Rutford Ice Streams, that are the greatest contributors to potential SLR (161 mm) and to simulation uncertainty (420 mm 6σ spread).

scholarly journals Stopping the Flood: Could We Use Targeted Geoengineering to Mitigate Sea Level Rise?

2018 ◽  
Author(s):  
Michael J. Wolovick ◽  
John C. Moore
Keyword(s):  
Sea Level Rise ◽  
Sea Level ◽  
Ice Sheet ◽  
Dynamic Feedback ◽  
West Antarctica ◽  
Ice Shelf ◽  
Long Run ◽  
Grounding Line ◽  
Individual Source ◽  
Engineering Projects

Abstract. The Marine Ice Sheet Instability (MISI) is a dynamic feedback that can cause an ice sheet to enter a runaway collapse. Thwaites Glacier, West Antarctica, is the largest individual source of future sea level rise and may have already entered the MISI. Here, we use a suite of coupled ice–ocean flowband simulations to explore whether targeted geoengineering using an artificial sill or artificial ice rises could counter a collapse. Successful interventions occur when the floating ice shelf regrounds on the pinning points, increasing buttressing and reducing ice flux across the grounding line. Regrounding is more likely with a continuous sill that is able to block warm water transport to the grounding line. The smallest design we consider is comparable in scale to existing civil engineering projects but has only a 30 % success rate, while larger designs are more effective. There are multiple possible routes forward to improve upon the designs that we considered, and with decades or more to research designs it is plausible that the scientific community could come up with a plan that was both effective and achievable. While reducing emissions remains the short-term priority for minimizing the effects of climate change, in the long run humanity may need to develop contingency plans to deal with an ice sheet collapse.

Deriving a Physically-Based Calving Rate Law for Marine Ice-Cliff Instability

2020 ◽  
Author(s):  
Anna Crawford ◽  
Joe Todd ◽  
Doug Benn ◽  
Jan Åström ◽  
Thomas Zwinger
Keyword(s):  
Viscous Flow ◽  
Sea Level Rise ◽  
Sea Level ◽  
Large Scale ◽  
Ice Sheet ◽  
Rate Law ◽  
Grounding Line ◽  
Physically Based ◽  
Calving Rate ◽  
Global Sea Level

<p><span>Rapid grounding line retreat at marine-terminating glaciers could expose ice cliffs with heights greater than those on observational record. However, the finite strength of ice places a limit on the height of subareal cliffs. It is proposed that marine ice-cliff instability (MICI) will begin once a stable height threshold is exceeded. If a glacier is situated over a retrograde slope, as is the case for Thwaites Glacier and much of the West Antarctic Ice Sheet, MICI can be expected to accelerate </span><span>as retreat progresses</span><span> and increasingly tall and unstable ice cliffs are </span><span>formed. This is consequential for global sea level rise, yet large uncertainties remain in the prediction of MICI retreat rates. </span></p><p><span>We investigate MICI by pairing the full Stokes continuum model Elmer/Ice and the Helsinki Discrete Element Model (HiDEM)</span><span>. Viscous flow, simulated in Elmer/Ice, is found to be a necessary pre-condition for MICI collapse. Forward advance and bulging lead to ice-front instability and pervasive crevassing in HiDEM. This culminates in full-thickness calving events. We do not observe calving at ice faces prior to viscous deformation. </span><span>HiDEM simulations that implement viscous flow (HiDEM-ve) also show forward advance and waterline bulging, similar to the Elmer/Ice simulations. However, the importance of granular shear is highlighted by pronounced shear bands and patterns of surface lowering in HiDEM-ve output. These results emphasize the importance and complexity of viscous and brittle process interaction during MICI.</span></p><p><span>A simulation matrix of grounded termini shows that calving frequency and magnitude increase with the thickness of the calving front. The time required for viscous flow to recreate unstable conditions is influenced by thickness as well as ice temperature and basal friction. Simulations of buoyant termini are seen to calve through basal-crevassing and block-rotation, as opposed to incising surface-crevasses. Lastly, we observe that b</span><span>uttressing </span><span>mélange can</span><span> suppress retreat rate</span><span> if a sufficient resistive force is delivered to the calving front. A </span><span>physically-based law for MICI retreat rate is derived from our simulation matrix; this calving rate law can be incorporated into large-scale ice sheet models to constrain projections of Antarctic retreat and associated global sea level rise. Our results will also be used to investigate the future retreat of Thwaites Glacier, which is vulnerable to MICI due to a retreating grounding line, fragile floating ice shelf, and precarious positioning above an overdeepening basin</span><span>. </span></p>

scholarly journals Greenland ice sheet contribution to sea-level rise from a new-generation ice-sheet model

2012 ◽  
Vol 6 (6) ◽  
pp. 1561-1576 ◽  
Author(s):  
F. Gillet-Chaulet ◽  
O. Gagliardini ◽  
H. Seddik ◽  
M. Nodet ◽  
G. Durand ◽  
...  
Keyword(s):  
Mass Balance ◽  
Sea Level Rise ◽  
Sea Level ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Surface Mass Balance ◽  
Ice Flow ◽  
Surface Mass ◽  
Continental Scale ◽  
New Generation

Abstract. Over the last two decades, the Greenland ice sheet (GrIS) has been losing mass at an increasing rate, enhancing its contribution to sea-level rise (SLR). The recent increases in ice loss appear to be due to changes in both the surface mass balance of the ice sheet and ice discharge (ice flux to the ocean). Rapid ice flow directly affects the discharge, but also alters ice-sheet geometry and so affects climate and surface mass balance. Present-day ice-sheet models only represent rapid ice flow in an approximate fashion and, as a consequence, have never explicitly addressed the role of ice discharge on the total GrIS mass balance, especially at the scale of individual outlet glaciers. Here, we present a new-generation prognostic ice-sheet model which reproduces the current patterns of rapid ice flow. This requires three essential developments: the complete solution of the full system of equations governing ice deformation; a variable resolution unstructured mesh to resolve outlet glaciers and the use of inverse methods to better constrain poorly known parameters using observations. The modelled ice discharge is in good agreement with observations on the continental scale and for individual outlets. From this initial state, we investigate possible bounds for the next century ice-sheet mass loss. We run sensitivity experiments of the GrIS dynamical response to perturbations in climate and basal lubrication, assuming a fixed position of the marine termini. We find that increasing ablation tends to reduce outflow and thus decreases the ice-sheet imbalance. In our experiments, the GrIS initial mass (im)balance is preserved throughout the whole century in the absence of reinforced forcing, allowing us to estimate a lower bound of 75 mm for the GrIS contribution to SLR by 2100. In one experiment, we show that the current increase in the rate of ice loss can be reproduced and maintained throughout the whole century. However, this requires a very unlikely perturbation of basal lubrication. From this result we are able to estimate an upper bound of 140 mm from dynamics only for the GrIS contribution to SLR by 2100.

scholarly journals Simulating the Antarctic ice sheet in the Late-Pliocene warm period: PLISMIP-ANT, an ice-sheet model intercomparison project

2014 ◽  
Vol 8 (6) ◽  
pp. 5539-5588 ◽  
Author(s):  
B. de Boer ◽  
A. M. Dolan ◽  
J. Bernales ◽  
E. Gasson ◽  
H. Goelzer ◽  
...  
Keyword(s):  
Sea Level Rise ◽  
Sea Level ◽  
Ice Sheets ◽  
Ice Sheet ◽  
Warm Period ◽  
Antarctic Ice Sheet ◽  
Late Pliocene ◽  
Bedrock Topography ◽  
Intercomparison Project ◽  
The Antarctic

Abstract. In the context of future climate change, understanding the nature and behaviour of ice sheets during warm intervals in Earth history is of fundamental importance. The Late-Pliocene warm period (also known as the PRISM interval: 3.264 to 3.025 million years before present) can serve as a potential analogue for projected future climates. Although Pliocene ice locations and extents are still poorly constrained, a significant contribution to sea-level rise should be expected from both the Greenland ice sheet and the West and East Antarctic ice sheets based on palaeo sea-level reconstructions. Here, we present results from simulations of the Antarctic ice sheet by means of an international Pliocene Ice Sheet Modeling Intercomparison Project (PLISMIP-ANT). For the experiments, ice-sheet models including the shallow ice and shelf approximations have been used to simulate the complete Antarctic domain (including grounded and floating ice). We compare the performance of six existing numerical ice-sheet models in simulating modern control and Pliocene ice sheets by a suite of four sensitivity experiments. Ice-sheet model forcing fields are taken from the HadCM3 atmosphere–ocean climate model runs for the pre-industrial and the Pliocene. We include an overview of the different ice-sheet models used and how specific model configurations influence the resulting Pliocene Antarctic ice sheet. The six ice-sheet models simulate a comparable present-day ice sheet, although the models are setup with their own parameter settings. For the Pliocene simulations using the Bedmap1 bedrock topography, some models show a small retreat of the East Antarctic ice sheet, which is thought to have happened during the Pliocene for the Wilkes and Aurora basins. This can be ascribed to either the surface mass balance, as the HadCM3 Pliocene climate shows a significant increase over the Wilkes and Aurora basin, or the initial bedrock topography. For the latter, our simulations with the recently published Bedmap2 bedrock topography indicate a significantly larger contribution to Pliocene sea-level rise from the East Antarctic ice sheet for all six models relative to the simulations with Bedmap1.

The influence of the solid Earth on the contribution of marine sections of the Antarctic ice sheet to future sea level change

2021 ◽  
Author(s):  
Maryam Yousefi ◽  
Jeannette Wang ◽  
Linda Pan ◽  
Natalya Gomez ◽  
Konstantin Latychev ◽  
...  
Keyword(s):  
Sea Level Rise ◽  
Solid Earth ◽  
Sea Level ◽  
Ice Sheet ◽  
Sea Level Change ◽  
Earth Structure ◽  
Level Change ◽  
Grounding Line ◽  
Level Model ◽  
The Antarctic

<p>The future retreat of marine-based sectors of the Antarctic Ice Sheet (AIS) and its consequent global mean sea level (GMSL) rise is driven by various climatic and non-climatic feedbacks between ice, ocean, atmosphere, and solid Earth. The primary mode of ice loss in marine sectors of the AIS is dynamic flow of ice across the grounding line into the ocean. The flux of ice across the grounding line is strongly sensitive to the thickness of ice there, which is in turn proportional to the water depth (sea level) such that sea level rise enhances ice loss and grounding line retreat while sea level fall acts to slow or stop migration of the grounding line. In response to the unloading from removal of ice mass, the underlying bedrock deforms isostatically leading to lower local sea surface which promotes stabilization of the grounding line. In addition to its effect on AIS evolution, solid Earth deformation also alters the shape and size of the ocean basin areas that are exposed as marine areas of ice retreat and influences the amount of meltwater that leaves Antarctica and contributes to global sea-level rise. The solid Earth deformational response to surface loading changes, in terms of both magnitude and timescales, depends on Earth rheology. Seismic tomography models indicate that the interior structure of the Earth is highly variable over the Antarctica with anomalously low shallow mantle viscosities across the western section of the AIS. An improved projection of the contribution from AIS to sea level change requires a consideration of this complexity in Earth structure. Here we adopt a state-of-the-art seismic velocity model to build a high-resolution 3D viscoelastic structure model beneath Antarctica. We incorporate this structure into a high spatiotemporal resolution sea-level model to simulate the influence of solid Earth deformation on contributions of the AIS evolution to future sea-level change. Our sea-level model is coupled with the dynamics of PSU ice sheet model and our calculations are based on a range of future climate forcings. We show that the influence of applying a spatially variable Earth structure is significant, particularly in the regions of West Antarctica where upper mantle viscosities are lower and the elastic lithosphere is thinned.</p>

scholarly journals Aurora Basin, the weak underbelly of East Antarctica

2020 ◽  
Author(s):  
Tyler Pelle ◽  
Mathieu Morlighem ◽  
Felicity S. McCormack
Keyword(s):  
Mass Balance ◽  
Sea Level Rise ◽  
Sea Level ◽  
Ice Sheet ◽  
Emission Scenarios ◽  
Thermal Forcing ◽  
Surface Mass ◽  
Grounding Line ◽  
Glacial Discharge ◽  
Global Sea Level

<p>Containing ~52 m sea level rise equivalent ice mass (SLRe), the East Antarctic Ice Sheet (EAIS) is a major component of the global sea level budget; yet, uncertainty remains in how this ice sheet will respond to enhanced atmospheric and oceanic thermal forcing through the turn of the century. To address this uncertainty, we model the most dynamic catchments of EAIS out to 2100 using the Ice Sheet System Model. We employ three basal melt rate parameterizations to resolve ice-ocean interactions and force our model with anomalies in both surface mass balance and ocean thermal forcing from both CMIP5 and CMIP6 model output. We find that this sector of EAIS gains approximately 10 mm SLRe by 2100 under high emission scenarios (RCP8.5 and SSP585), and loses mass under low emission scenarios (RCP2.6). All basins within the domain either gain mass or are in near mass balance through the 86-year experimental period, except the Aurora Subglacial Basin. The primary region of mass loss in this basin is located within 50 km upstream of Totten Glacier’s grounding line, which loses up to 6 mm SLRe by 2100. Glacial discharge from Totten is modulated by buttress supplied by a 10 km ice plain, located along the southern-most end of Totten’s grounding line. This ice plain is sensitive to brief changes in ocean temperature and once ungrounded, glacial discharge from Totten accelerates by up to 70% of it present day configuration. In all, we present plausible bounds on the contribution of a large sector of EAIS to global sea level rise out to the end of the century and target Totten as the most vulnerable glacier in this region. In doing so, we reduce uncertainty in century-scale global sea level projections and help steer scientific focus to the most dynamic regions of EAIS.</p>

scholarly journals The GRISLI-LSCE contribution to ISMIP6, Part 2: projections of the Antarctic ice sheet evolution by the end of the 21<sup>st</sup> century

2020 ◽  
Author(s):  
Aurélien Quiquet ◽  
Christophe Dumas
Keyword(s):  
Sea Level Rise ◽  
Sea Level ◽  
Greenhouse Gas Emission ◽  
Ice Sheet ◽  
Companion Paper ◽  
Antarctic Ice Sheet ◽  
Grounding Line ◽  
Common Framework ◽  
Global Sea Level ◽  
The Antarctic

Abstract. Of primary societal importance, the ice sheet contribution to global sea level rise over the 21st century remains largely uncertain. In particular, the contribution of the Antarctic ice sheet by 2100 ranges from a few millimetres to more than one metre in the recent literature. The Ice Sheet Model Intercomparison Project for CMIP6 aimed at reducing the uncertainties on the fate of the ice sheets in the future by gathering various ice sheet models in a common framework. While in a companion paper we present the GRISLI-LSCE contribution to ISMIP6-Greenland, we present here the GRISLI-LSCE contribution to ISMIP6-Antarctica. We show that our model is strongly sensitive to the climate forcing used, with a contribution of the Antarctic ice sheet to global sea level rise by 2100 that ranges from −50 mm to +150 mm of sea level equivalent. Future oceanic warming leads to a decrease in thickness of the ice shelves and implies grounding line retreats while increased precipitation partially mitigates the ice sheet contribution to global sea level rise. Most of ice sheet changes over the next century are dampened under low greenhouse gas emission scenarios. Uncertainties related to sub-shelf basal melt induce large differences in simulated grounding line retreats, confirming the importance of this process and its representation in ice sheet models for the projections of the Antarctic ice sheet.

scholarly journals Greenland Ice Sheet contribution to sea-level rise from a new-generation ice-sheet model

2012 ◽  
Vol 6 (4) ◽  
pp. 2789-2826 ◽  
Author(s):  
F. Gillet-Chaulet ◽  
O. Gagliardini ◽  
H. Seddik ◽  
M. Nodet ◽  
G. Durand ◽  
...  
Keyword(s):  
Mass Balance ◽  
Sea Level Rise ◽  
Sea Level ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Surface Mass Balance ◽  
Ice Flow ◽  
Surface Mass ◽  
Continental Scale ◽  
New Generation

Abstract. Over the last two decades, the Greenland Ice Sheet (GrIS) has been losing mass at an increasing rate, enhancing its contribution to sea-level rise. The recent increases in ice loss appear to be due to changes in both the surface mass balance of the ice sheet and ice discharge (ice flux to the ocean). Rapid ice flow directly affects the discharge, but also alters ice-sheet geometry and so affects climate and surface mass balance. The most usual ice-sheet models only represent rapid ice flow in an approximate fashion and, as a consequence, have never explicitly addressed the role of ice discharge on the total GrIS mass balance, especially at the scale of individual outlet glaciers. Here, we present a new-generation prognostic ice-sheet model which reproduces the current patterns of rapid ice flow. This requires three essential developments: the complete solution of the full system of equations governing ice deformation; an unstructured mesh to usefully resolve outlet glaciers and the use of inverse methods to better constrain poorly known parameters using observations. The modelled ice discharge is in good agreement with observations on the continental scale and for individual outlets. By conducting perturbation experiments, we investigate how current ice loss will endure over the next century. Although we find that increasing ablation tends to reduce outflow and on its own has a stabilising effect, if destabilisation processes maintain themselves over time, current increases in the rate of ice loss are likely to continue.

scholarly journals Layered seawater intrusion and melt under grounded ice

2021 ◽  
Author(s):  
Alexander A. Robel ◽  
Earle Wilson ◽  
Helene Seroussi
Keyword(s):  
Sea Level Rise ◽  
Sea Level ◽  
Seawater Intrusion ◽  
Ice Sheets ◽  
Ice Sheet ◽  
System Model ◽  
Glacier Terminus ◽  
Grounding Line ◽  
Flow Through ◽  
And Sea Level

Abstract. Increasing melt of ice sheets at their floating or vertical interface with the ocean is a major driver of marine ice sheet retreat and sea level rise. However, the extent to which warm, salty seawater may drive melting under the grounded portions of ice sheets is still not well understood. Previous work has explored the possibility that dense seawater intrudes beneath relatively light subglacial freshwater discharge, similar to the salt wedge observed in many estuarine systems. In this study, we develop a generalized theory of layered seawater intrusion under grounded ice, including where subglacial hydrology occurs as a macroporous water sheet over impermeable beds or as microporous Darcy flow through permeable till. Using predictions from this theory, we show that seawater intrusion over hard beds may feasibly occur up to tens of kilometers upstream of a glacier terminus or grounding line. On the other hand, seawater is unlikely to intrude more than tens of meters through subglacial till. High-resolution simulations using the Ice-Sheet and Sea-Level System Model (ISSM) show that even just a few hundred meters of basal melt caused by seawater intrusion upstream of marine ice sheet grounding lines can cause projections of marine ice sheet volume loss to be 10–50 % higher or 100 % higher for kilometers of intrusion-induced basal melt. These results suggest that further observational, experimental and numerical investigations are needed to determine whether the conditions under which extensive seawater intrusion occurs and whether it will indeed drive rapid marine ice sheet retreat and sea level rise in the future.

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